Single Network Slice Selection Assistance Information

Description

The Single Network Slice Selection Assistance Information (S-NSSAI) is a fundamental construct in the 5G System (5GS) that uniquely identifies a network slice. A network slice is a logical, end-to-end network tailored to meet the specific requirements of a service or customer. The S-NSSAI is used by the User Equipment (UE) and the network to select and associate the UE with the appropriate network slice instance during registration and session establishment procedures. It is a critical parameter carried in Non-Access Stratum (NAS) signaling messages between the UE and the Access and Mobility Management Function (AMF).

An S-NSSAI is composed of two parts: a mandatory Slice/Service Type (SST) and an optional Slice Differentiator (SD). The SST is an 8-bit value that indicates the expected network slice behavior in terms of features and services. Standardized SST values include 1 for enhanced Mobile Broadband (eMBB), 2 for Ultra-Reliable Low Latency Communications (URLLC), and 3 for massive IoT (mIoT). The SD is a 24-bit optional identifier used to differentiate among multiple network slices of the same SST, allowing operators to create further specialized slices within a broad category (e.g., different eMBB slices for enterprise and consumer services). The combination of SST and SD allows for fine-grained slice selection.

During initial registration, the UE provides a Requested NSSAI, which is a list of S-NSSAIs corresponding to the slices it wishes to access. The network validates these requests against the subscriber's Subscribed S-NSSAIs stored in the Unified Data Management (UDM). The AMF, in coordination with the Network Slice Selection Function (NSSF), selects the appropriate network slice instances and returns an Allowed NSSAI to the UE for use in the current registration area. This Allowed NSSAI is then used for subsequent session establishment with the Session Management Function (SMF). The S-NSSAI influences the selection of all other core network functions (SMF, PCF, UPF) and can be used to apply specific network policies, quality of service (QoS) profiles, and charging rules, enabling true end-to-end logical network isolation and customization.

Purpose & Motivation

S-NSSAI was introduced in 3GPP Release 15 as a core enabler of 5G network slicing, a revolutionary concept that allows a single physical network infrastructure to be partitioned into multiple virtual, independent logical networks. Prior to 5G, networks provided largely monolithic services; customizing the network for different service types (e.g., video streaming, autonomous driving, sensor networks) was complex and inefficient. S-NSSAI solves this by providing a simple, standardized identifier that allows both the device and the network to dynamically select a pre-configured slice with specific characteristics.

The creation of S-NSSAI was motivated by the diverse and stringent requirements of 5G use cases, which range from high bandwidth and low latency to massive connection density. It addresses the problem of how to efficiently direct traffic and apply tailored resources without building separate physical networks. By including the S-NSSAI in signaling, the 5G system can instantiate the correct set of network functions and policies on a per-session basis. This enables operators to offer Network-as-a-Service (NaaS), support enterprise private networks, and optimize resource utilization, thereby unlocking new revenue streams and meeting the performance demands of modern applications.